pekko/akka-distributed-data/src/main/scala/akka/cluster/ddata/VersionVector.scala

/**
 * Copyright (C) 2009-2015 Typesafe Inc. <http://www.typesafe.com>
 */
package akka.cluster.ddata

import java.util.concurrent.atomic.AtomicLong

import scala.annotation.tailrec
import scala.collection.immutable.TreeMap

import akka.cluster.Cluster
import akka.cluster.UniqueAddress

/**
 * VersionVector module with helper classes and methods.
 */
object VersionVector {

  /**
   * INTERNAL API
   */
  private[akka] val emptyVersions: TreeMap[UniqueAddress, Long] = TreeMap.empty
  val empty: VersionVector = new VersionVector(emptyVersions)
  def apply(): VersionVector = empty
  /**
   * Java API
   */
  def create(): VersionVector = empty

  sealed trait Ordering
  case object After extends Ordering
  case object Before extends Ordering
  case object Same extends Ordering
  case object Concurrent extends Ordering
  /**
   * Marker to ensure that we do a full order comparison instead of bailing out early.
   */
  private case object FullOrder extends Ordering

  /**
   * Java API: The `VersionVector.After` instance
   */
  def AfterInstance = After

  /**
   * Java API: The `VersionVector.Before` instance
   */
  def BeforeInstance = Before

  /**
   * Java API: The `VersionVector.Same` instance
   */
  def SameInstance = Same

  /**
   * Java API: The `VersionVector.Concurrent` instance
   */
  def ConcurrentInstance = Concurrent

  private object Timestamp {
    final val Zero = 0L
    final val EndMarker = Long.MinValue
    val counter = new AtomicLong(1L)
  }

  /**
   * Marker to signal that we have reached the end of a version vector.
   */
  private val cmpEndMarker = (null, Timestamp.EndMarker)

}

/**
 * Representation of a Vector-based clock (counting clock), inspired by Lamport logical clocks.
 * {{{
 * Reference:
 *    1) Leslie Lamport (1978). "Time, clocks, and the ordering of events in a distributed system". Communications of the ACM 21 (7): 558-565.
 *    2) Friedemann Mattern (1988). "Virtual Time and Global States of Distributed Systems". Workshop on Parallel and Distributed Algorithms: pp. 215-226
 * }}}
 *
 * Based on code from `akka.cluster.VectorClock`.
 *
 * This class is immutable, i.e. "modifying" methods return a new instance.
 */
@SerialVersionUID(1L)
final case class VersionVector private[akka] (
  private[akka] val versions: TreeMap[UniqueAddress, Long])
  extends ReplicatedData with ReplicatedDataSerialization with RemovedNodePruning {

  type T = VersionVector

  import VersionVector._

  /**
   * Increment the version for the node passed as argument. Returns a new VersionVector.
   */
  def +(node: Cluster): VersionVector = increment(node)

  /**
   * INTERNAL API
   * Increment the version for the node passed as argument. Returns a new VersionVector.
   */
  private[akka] def +(node: UniqueAddress): VersionVector = increment(node)

  /**
   * Increment the version for the node passed as argument. Returns a new VersionVector.
   */
  def increment(node: Cluster): VersionVector = increment(node.selfUniqueAddress)

  /**
   * INTERNAL API
   * Increment the version for the node passed as argument. Returns a new VersionVector.
   */
  private[akka] def increment(node: UniqueAddress): VersionVector =
    copy(versions = versions.updated(node, Timestamp.counter.getAndIncrement()))

  /**
   * Returns true if <code>this</code> and <code>that</code> are concurrent else false.
   */
  def <>(that: VersionVector): Boolean = compareOnlyTo(that, Concurrent) eq Concurrent

  /**
   * Returns true if <code>this</code> is before <code>that</code> else false.
   */
  def <(that: VersionVector): Boolean = compareOnlyTo(that, Before) eq Before

  /**
   * Returns true if <code>this</code> is after <code>that</code> else false.
   */
  def >(that: VersionVector): Boolean = compareOnlyTo(that, After) eq After

  /**
   * Returns true if this VersionVector has the same history as the 'that' VersionVector else false.
   */
  def ==(that: VersionVector): Boolean = compareOnlyTo(that, Same) eq Same

  /**
   * Version vector comparison according to the semantics described by compareTo, with the ability to bail
   * out early if the we can't reach the Ordering that we are looking for.
   *
   * The ordering always starts with Same and can then go to Same, Before or After
   * If we're on After we can only go to After or Concurrent
   * If we're on Before we can only go to Before or Concurrent
   * If we go to Concurrent we exit the loop immediately
   *
   * If you send in the ordering FullOrder, you will get a full comparison.
   */
  private final def compareOnlyTo(that: VersionVector, order: Ordering): Ordering = {
    def nextOrElse[A](iter: Iterator[A], default: A): A = if (iter.hasNext) iter.next() else default

    def compare(i1: Iterator[(UniqueAddress, Long)], i2: Iterator[(UniqueAddress, Long)], requestedOrder: Ordering): Ordering = {
      @tailrec
      def compareNext(nt1: (UniqueAddress, Long), nt2: (UniqueAddress, Long), currentOrder: Ordering): Ordering =
        if ((requestedOrder ne FullOrder) && (currentOrder ne Same) && (currentOrder ne requestedOrder)) currentOrder
        else if ((nt1 eq cmpEndMarker) && (nt2 eq cmpEndMarker)) currentOrder
        // i1 is empty but i2 is not, so i1 can only be Before
        else if (nt1 eq cmpEndMarker) { if (currentOrder eq After) Concurrent else Before }
        // i2 is empty but i1 is not, so i1 can only be After
        else if (nt2 eq cmpEndMarker) { if (currentOrder eq Before) Concurrent else After }
        else {
          // compare the nodes
          val nc = nt1._1 compareTo nt2._1
          if (nc == 0) {
            // both nodes exist compare the timestamps
            // same timestamp so just continue with the next nodes
            if (nt1._2 == nt2._2) compareNext(nextOrElse(i1, cmpEndMarker), nextOrElse(i2, cmpEndMarker), currentOrder)
            else if (nt1._2 < nt2._2) {
              // t1 is less than t2, so i1 can only be Before
              if (currentOrder eq After) Concurrent
              else compareNext(nextOrElse(i1, cmpEndMarker), nextOrElse(i2, cmpEndMarker), Before)
            } else {
              // t2 is less than t1, so i1 can only be After
              if (currentOrder eq Before) Concurrent
              else compareNext(nextOrElse(i1, cmpEndMarker), nextOrElse(i2, cmpEndMarker), After)
            }
          } else if (nc < 0) {
            // this node only exists in i1 so i1 can only be After
            if (currentOrder eq Before) Concurrent
            else compareNext(nextOrElse(i1, cmpEndMarker), nt2, After)
          } else {
            // this node only exists in i2 so i1 can only be Before
            if (currentOrder eq After) Concurrent
            else compareNext(nt1, nextOrElse(i2, cmpEndMarker), Before)
          }
        }

      compareNext(nextOrElse(i1, cmpEndMarker), nextOrElse(i2, cmpEndMarker), Same)
    }

    if ((this eq that) || (this.versions eq that.versions)) Same
    else compare(this.versions.iterator, that.versions.iterator, if (order eq Concurrent) FullOrder else order)
  }

  /**
   * Compare two version vectors. The outcome will be one of the following:
   * <p/>
   * {{{
   *   1. Version 1 is SAME (==)       as Version 2 iff for all i c1(i) == c2(i)
   *   2. Version 1 is BEFORE (<)      Version 2 iff for all i c1(i) <= c2(i) and there exist a j such that c1(j) < c2(j)
   *   3. Version 1 is AFTER (>)       Version 2 iff for all i c1(i) >= c2(i) and there exist a j such that c1(j) > c2(j).
   *   4. Version 1 is CONCURRENT (<>) to Version 2 otherwise.
   * }}}
   */
  def compareTo(that: VersionVector): Ordering = {
    compareOnlyTo(that, FullOrder)
  }

  /**
   * Merges this VersionVector with another VersionVector. E.g. merges its versioned history.
   */
  def merge(that: VersionVector): VersionVector = {
    var mergedVersions = that.versions
    for ((node, time) ← versions) {
      val mergedVersionsCurrentTime = mergedVersions.getOrElse(node, Timestamp.Zero)
      if (time > mergedVersionsCurrentTime)
        mergedVersions = mergedVersions.updated(node, time)
    }
    VersionVector(mergedVersions)
  }

  override def needPruningFrom(removedNode: UniqueAddress): Boolean =
    versions.contains(removedNode)

  override def prune(removedNode: UniqueAddress, collapseInto: UniqueAddress): VersionVector =
    copy(versions = versions - removedNode) + collapseInto

  override def pruningCleanup(removedNode: UniqueAddress): VersionVector = copy(versions = versions - removedNode)

  override def toString = versions.map { case ((n, t)) ⇒ n + " -> " + t }.mkString("VersionVector(", ", ", ")")
}